A typical example of the dog clutch mechanisms of the type concerned is disclosed in Japanese Patent Laid-open Publication (JP-A) No. 2005-48820, which corresponds to U.S. Pat. No. 6,893,305 patented on May 17, 2005. As shown in FIG. 7 hereof, the disclosed dog clutch mechanism includes a forward gear 101 rotatably mounted on a propeller shaft, not shown, the forward gear 101 having teeth 102 on a surface that opposes one end face of a clutch dog member 103. The clutch dog member 103 is slidably mounted on the non-illustrated propeller shaft and has recesses 104 cut into the end face for meshing engagement with the teeth 102 of the forward gear 101. The forward gear 101 is continuously driven when an engine, not shown, is running. With this arrangement, when the clutch dog member 103 is axially displaced in a direction to move the recesses 104 into meshing engagement with the teeth 102 of the forward gear 101, power from the engine is transmitted from the forward gear 101 to the clutch dog member 103, causing the propeller shaft to rotate together with the clutch dog member 103.
In the known dog clutch mechanism, the circumferential length L1 of each of the recesses 104 of the clutch dog member 103 and the circumferential length L2 of each of the teeth 102 of the forward gear 101 are set to be substantially equal to each other so that a clearance formed between each recess 104 and a mating one of the teeth 102 during engagement between the clutch dog member 103 and the forward gear 101 is made as small as possible. The thus formed circumferential clearance is small, but the presence of the circumferential clearance still allows the teeth 102 and the recesses 104 to strike together and hence generate striking noise when the clutch dog member 103 and the forward gear 101 are subjected to torsional vibrations produced when the engine undergoes irregular combustion. Furthermore, the teeth 102 and the recesses 104 require high dimensional accuracies in terms of the dimensions in the circumferential direction, which will increase production cost the dog clutch mechanism.
It is accordingly an object of the present invention to provide a dog clutch mechanism for outboard motors, which can operate silently without involving generation of striking noise and can be manufactured at a relatively low cost.